This invention relates to electic resistance heaters and more particularly to cartridge heaters.
This invention is directly concerned with a heater of the type described in the coassigned Desloge U.S. Pat. No. 2,831,951 which has an electric resistance heating wire wound around a cylindric ceramic core to form a heating element. The core typically has two or more longitudinal bores therein. One end of the resistance heating wire (referred to as a pigtail) is inserted in one of the bores and the other end or pigtail of the heating wire is inserted in the other of the bores at the opposite end of the core. A terminal wire or pin is then inserted in each bore from the end thereof which has a pigtail inserted therein and the resulting assembly is then inserted in a tubular metal sheath of somewhat larger diameter than the heating element. A particulate electrical insulative material, such as magnesium oxide (MgO) or the like, is poured into the open end of the sheath to fill the annular space between the heating element and the inner surface of the sheath. Appropriate seals are placed at the ends of the sheath and the entire assembly is compacted or compressed, as by swaging or by other suitable process, to reduce the diameter of the sheath and to thus compact and compress the MgO and to at least partially crush the ceramic core so as to collapse the core about the pins to insure good electrical contact between the pigtails and the pins. The compacted MgO provides a relatively good heat transfer path between the heating element and the sheath and it also electrically insulates the sheath from the heating element.
It has long been a problem to fabricate cartridge heaters with a layer of compacted particulate insulative material of uniform thickness along the entire length of the heating element. These variations in thickness of the insulative layer between the heating element and the sheath have caused problems. Ideally, the insulative material should be as thin as possible to enhance heat transfer, but yet should be sufficiently thick to reliably, electrically insulate the heating element from the sheath. If only one point on the electrical resistance wire coil touches the metal sheath, that heating element is shorted thus making it inoperative. If the layer of compacted insulative material is thicker than it need be to insure electrical insulation for the heating element, the electrical resistance heating wire must be operated at a higher temperature level to yield a desired heat output for the heater. This higher operating temperature for the electrical resistance wire may, however, cause the wire to more rapidly oxidize or to otherwise deteriorate and thus may considerably shorten the life of the heater.
In addition to the above-mentioned U.S. Pat. No. 2,831,951, other references, such as the coassigned U.S. Pat. Nos. 3,582,616 and 3,839,623, may be referred to which show other cartridge heaters broadly similar to the heater of this invention.